A handover (HO) indicates that a mobile station moves from a radio interface in a base station to a radio interface of a different base station. A handover process in a general IEEE 802.16 system is described in the following description.
FIG. 1 indicates an example of performing a handover procedure from a serving base station (S-ABS) to a target base station (T-ABS) by an advanced mobile station (AMS).
Referring to FIG. 1, while performing a data exchange with a serving base station [S101], a mobile station sends a handover request message (AAI-HO-REQ) to the serving base station when a handover trigger condition is satisfied [S102].
According to the message, the serving base station exchanges information with a target base station and sends a handover command message (AAI-HO-CMD) including information necessary for making a handover to the target base station to the mobile station [S103].
The mobile station tries a CDMA ranging to the target base station using a CDMA raging code [S104].
In case that the target base station have successfully received the ranging code, which is transmitted by the mobile station, the target base station sends a corresponding code and a raging response message (AAI-RNG-ACK) including raging status information on the corresponding code to the mobile station [S105]. The ranging status information is included in the ranging response message in a form of a ranging status field.
In case that the ranging status field is set to ‘success’, the mobile station exchanges a ranging request/response message with the target base station [S106, S107] and may be then able to normally perform data exchange with the target base station in a manner of completing a network re-entry process to the target base station [S108].
The aforementioned handover procedure assumed to be performed between a mobile station and a base station following IEEE 802.16m (wirelessMAN-advanced air interface) standard. This system is a more advanced system than a conventional IEEE 802.16e (wirelessMAN-OFDMA R1 reference system). The handover process defined by the two systems may have a different kind of medium access control management message and parameters belong thereto.
For clarity of the present specification, a system to which a general technology including IEEE 802.16e standard applied is called a legacy system or R1 system in the following description. And, a mobile station to which a legacy technology is applied is called a legacy mobile station or an R1 MS. A base station to which the legacy technology is applied is called a legacy base station or an R1 BS. Moreover, an operating mode of the mobile station or the base station to which a general technology is applied is called a legacy mode.
And, a mobile station to which a technology including IEEE 802.16m standard more advanced than the general technology is called an AMS (advanced MS) or an advanced mobile station. A base station to which the advanced technology is applied is called an ABS (advanced BS) or an advanced base station. Moreover, an operating mode of the mobile station or the base station to which the advanced technology is applied is called an advanced mode.
Assume that the AMS receives a service in a manner of accessing an YBS and an ABS (wirelessMAN-OFDMA R1 reference system/wirelessMAN-OFDMA advanced co-existing system) supporting both the AMS and the YMS exists near the YBS. In the following description, both the AMS and the YMS supportive of, i.e., both the legacy mode and the advanced mode supportive of the ABS is called a mix mode base station and an operating mode of the base station is called a mix mode.
Assume that the YBS only has a legacy zone (LZone: legacy zone) having a physical channel frame structure to which applied a legacy system and the ABS only has an advanced mobile station supporting zone (MZone: 16MZone) having a physical frame channel structure to which applied an advanced system (wirelessMAN-OFDMA advanced system only) in case that the ABS supports the AMS only. And, assume that both the AMS and the YMS supportive of the ABS (wirelessMAN-OFDMA R1 reference system/wirelessMAN-OFDMA advanced co-existing system: legacy supportive) has both the legacy zone and the advanced mobile station supporting zone and is distinguished by a time unit, for instance, a frame or a subframe unit in uplink and downlink (TDD: time division duplex),respectively.
Moreover, assume that the AMS is able to receive a service from both the ABS and the YBS. In particular, assume that the AMS is able to receive the service from one of the advanced mobile station supporting zone and the legacy zone and the AMS is able to perform both the handover performing procedure defined by the legacy system and the handover performing procedure defined by the advanced system.
In general, in order to make a handover from a serving YBS to the ABS supporting the AMS and the YMS, the AMS enters the legacy zone of the ABS first and then continuously receives a service in the legacy zone. Or, the AMS may be able to perform a zone switch to the advanced mobile station supporting zone. And, the AMS may be able to perform a handover in a manner of switching a zone to the advanced mobile station supporting zone without entering the legacy zone of the ABS.
At this time, regarding the zone switch, the zone switch corresponds to a process enabling the AMS operating in the LZone to operate in a resource region of the MZone, since the LZone and the MZone distinguished by a time division duplex (TDD) exist in a single carrier. In particular, it means that a mobile station performs the MAC operation defined by IEEE 802.16m system in the MAC operation defined by IEEE 802.16e system. This also includes that the AMS inversely moves from the MZone to the LZone.
Yet, in order for a mobile station to perform a zone switch in a same base station when the zone switch is performed, the mobile station and a target base station should be aware of some information in advance. For instance, the target base station should be aware of whether the mobile station is IEEE 802.16m system supportive of mobile station to schedule a handover or a zone changing process to be performed in accordance with the information. And, the mobile station should be aware of the information on the zone to move, i.e., whether the target base station is IEEE 802.16m supportive of base station. Specifically, in case that the mobile station performs a zone switch from the LZone to the MZone, the mobile station should receive system information on the MZone (i.e., superframe header:SFH) and should be aware of the starting position of the MZone in a frame structure.
After a mobile station performed a handover defined by IEEE 802.16e system to the LZone of a mix mode base station, a method for performing a zone switch to the MZone is mainly classified into two methods. The method can be defined in a manner of dividing into a zone switch mode 0 and a zone switch mode 1. The zone switch mode 0 indicates that a mobile station disconnects a connection to the LZone on a specific time (for instance, a zone switch action time), synchronizes with the MZone and receives system information (SFH). Having finished the above mentioned successfully, the mobile station starts to enter a network to the MZone. At this time, latency as much as synchronization and SFH receiving time according to the network entry occurs.
On the contrary, in case of the zone switch mode 1, a mobile station may be able to simultaneously perform the synchronization and a network re-entry to the MZone while maintaining the operation performed in the LZone. Therefore, the latency may not occur in case of a zone switch.
In terms of latency, it is preferable to follow the zone switch mode 1. Yet, in terms of implementing of a mobile station, the zone switch mode 1 may be very demanding for the mobile station since the mobile station should simultaneously perform a communication with the two zones. On the contrary, in terms of implementing of the mobile station, it may be preferable to follow the zone switch mode 0. Yet, it may cause a significant impact on QoS due to a considerable latency. As mentioned in the above, it is because the mobile station should receive the system information on the MZone to perform a network entry to the MZone. Moreover, in case of performing a zone switch with the zone switch mode 0, since the mobile station does not know a frame configuration of the MZone and the starting point of a superframe, the mobile station may have a burden of performing the zone switch in a manner of scanning all sections of the MZone from a specific timing point (for instance, zone switch action time).